Custom versus fixed-percentage shrinkage rulers

If your clay tests at 13% shrinkage and a ruler offers 12% or 14%, is either close enough? I would want to know what that difference means on the finished piece before choosing another tool. A percentage sounds abstract; a dish that comes out nearly 2 mm smaller than planned is easier to judge.

A custom label doesn't make a scale more accurate. What matters is how closely its rate matches your tested clay, and whether the dimensional difference is small enough for this project.

What are you choosing between?

A fixed-percentage scale is made for a particular rate. A ruler can carry several fixed scales: DiamondCore's Shrink Ruler, for example, lists 6%, 10%, 12% and 14%. Those are that product's presets, not the options for every ruler.

A multi-rate set gives you several rates across separate rulers or scales. It can be useful when you move between clay bodies or firing conditions, provided it includes the rates you need. More scales do not automatically mean a closer match: a set containing 12% and 14% still leaves a 13% result between them.

A custom-percentage ruler has a scale made for a requested rate, subject to what its maker supports. Once made, it is still a fixed scale. It does not adjust itself when your clay or firing changes.

If a standard 13% ruler already matches your reliable 13% test result, a custom-made 13% ruler offers no extra numerical accuracy merely by being custom. Equally, selecting a rate from a shop's menu is not necessarily the same as being able to request any percentage.

Put the difference into millimetres

Here is a hypothetical comparison for a 150 mm glaze-fired length. Assume repeated tests of the same clay, at comparable wet working moisture and with the intended forming, drying and firing methods, support 13% linear shrinkage from wet to cooled glaze-fired. Linear means a change in length, not volume.

To isolate the choice of rate, the calculation assumes uniform shrinkage of exactly 13%. Real pieces will not be so obliging.

First calculate the wet length using the chosen ruler rate, then calculate what that length would become at the tested rate:

• Wet length = target glaze-fired length ÷ (1 − chosen rate ÷ 100)
• Glaze-fired length = wet length × (1 − tested rate ÷ 100)

The table is rounded to two decimal places to show the comparison, not to promise that level of making or measuring precision. Each result uses the unrounded wet length.

Using too low a rate makes the planned wet piece too small; using too high a rate makes it too large. The gaps from 12% to 13% and from 13% to 14% are each one percentage point, but they do not create exactly equal errors when working backwards from a target.

Old Forge Creations explains the reverse-sizing calculation, including why simply adding the shrinkage percentage to your target is not the same calculation.

How much difference can the piece accept?

A tolerance is the amount a finished measurement may differ from its target and still be acceptable. It belongs to the project, not to the ruler.

For the example above, suppose you would accept anything from 148 to 152 mm. Both neighbouring presets fall inside that range on paper, but only just:

• With the 12% scale, the predicted 148.30 mm piece could come out only about 0.30 mm smaller before falling below your lower limit.
• With the 14% scale, the predicted 151.74 mm piece could come out only about 0.26 mm larger before exceeding your upper limit.

That is not much room left for making and firing variation. A matching 13% scale puts the prediction in the middle of the range, but still cannot promise the result. If your acceptable range were 149 to 151 mm, neither neighbouring preset would pass even this idealised calculation. These are example limits, not recommended pottery tolerances.

I would be more relaxed about a small size difference on a standalone dish than on a piece intended to sit inside an existing rigid recess. Also ask whether being too large and too small have the same consequences: “nearest percentage” is not enough when only one direction of error is tolerable.

A better-matched rate removes one predictable source of difference. It does not remove measuring error, changes in working moisture, uneven drying or distortion. A part that must fit still needs a trial in the intended form. Plan clearance, the working gap between parts, separately, including any room needed for glaze; don't rely on a mismatched rate to create it.

Is your test result specific enough to justify the scale?

Before seeking a finer rate, check whether your measurements repeat. A single calculated result with several decimal places is not evidence that your process repeats to those decimal places.

Keep the starting and finishing stages consistent. A wet-to-glaze-fired percentage is not interchangeable with leather-hard-to-glaze-fired, wet-to-bone-dry or wet-to-bisque shrinkage. Record the clay, working moisture and firing conditions alongside the result. Digitalfire's SHAB testing method treats drying and firing shrinkage separately and stresses consistent sample preparation.

Our test-bar guide covers gathering that evidence. If repeated results differ enough to affect the intended fit, investigate that spread before treating a more specific scale as the answer. If the results are consistent but sit between available rates, compare the dimensional difference against your project's tolerance. The decimal-rate guide takes that particular rounding decision further.

You can also use the closer rate without buying another ruler. For the 13% example, mark the calculated 172.41 mm wet span with an ordinary ruler, to the precision you can actually measure, or make a wet-size template. A dedicated matching scale saves repeated calculation; it isn't the only way to keep the measured rate.

What that means for this ruler

The PoodlePotter preview lists whole-number rates from 10% through 18%, so the example's 13% rate is already a standard choice. It does not need a custom scale. Decimal percentages and rates outside that range are not confirmed.

If your measurements do call for a rate made specifically for you, the custom-scale guide helps you put together a brief for a maker.